Horn Coral - Rugosa 
Rugosans are often referred to as Horn corals because of a unique horn-shaped
The fossil is the skeleton of the coral animal or polyp. They built these cone shaped structures of calcium carbonate, where the animal lived at the top of the cone.
They had many tentacles sticking out the top to gather their food from the passing seawater.
Rugosa corals are found in rocks from the Ordovician Period. Many species evolved during the Paleozoic Era. During this time horn corals were so plentiful they helped to create reefs on the ocean floor.
As a group they flourished until the Permian Period when they became extinct along with most living things during the Great Permian Extinction, Roughly 252 million years ago.